94 SHO intermittent start problem

Issue is happening more often.Engine always cranks but sometimes stalls or starts lean or on less that six cylinders.Check engine light pops on and tachometer stops working.After letting it run for a little while or restarting several times it starts to run o.k. Later the check engine light goes off and the tach comes back on.Other than that the car runs perfectly.I have checked fuel pressure and it is normal.I always turn the key too on ,then crank the engine after fuel pump primes.It seems to happen when you don't
touch the gas pedal but i;m just starting to experiment with holding down the throttle slightly.Most of the time it starts and fires normally. Some codes come up (122,214,656,212,121) but are a result of this haywire control problem not from prior driving.Any ideas?

Welcome to the forum!

121 Closed throttle voltage higher or lower than expected
or 121 Indicates throttle position voltage inconsistent with the MAF sensor
122 Throttle Position (TP) sensor circuit below minimum voltage
212 Loss of Ignition Diagnostic Monitor (IDM) input to PCM/SPOUT circuit grounded
214 Cylinder Identification (CID) circuit failure
656 Torque Converter Clutch continuous slip error

I'd start by checking the Throttle Position Sensor (TPS) to make sure it responds linearly, and that the throttle plate is able to fully close when your foot is off the accelerator.

The cylinder identification code could be due to the camshaft position (CMP) sensor or possibly the Crankshaft Position (CKP) sensor. The CKP sensor is a common item to fail when the water pump leaks and will cause a no start or intermittent stalling when it fails on the Gen 1 and Gen 2 SHO engines.

-Rod

Thanks for your reply.I will consider and check the things you have said.I guess what throws me is that if the tpi or cpk have failed or are intermittent then i would see driveability problems at other times.I am trying starting the car with throttle slightly depressed and it fires without problems although today it tried to die once but since i had the throttle open in cleared up quickly .I know it should start without touching the throttle as designed.Let me know if you have any other ideasand is the loss of tachometer any clue? terry

what was the fuel pressure??
what engine is in car??

The loss of tachometer is most likely the "Cylinder identification" issue, and is probably due to either the CMP sensor or possibly the ignition module.

If your TPS sensor is non-linear, it could be giving a very odd reading when the throttle is closed, contributing to the 121 and 122 diagnostic codes. As you open the throttle, the TPS voltage gets back to the expected range and therefore your car starts fine. You can also try cleaning the throttle body, throttle plate, Idle Air Control (IAC) servo, and IAC passages. Issues with the IAC or passages could contribute to the problem starting as well as why it may start when the throttle is slightly depressed, but would not explain the 121 and 122 diagnostic codes.

The TPS is pretty easy to test if you have a multimeter. Testing the CMP correctly really will require an oscilloscope.

-Rod

Tpi tests normal although i will replace it anyway because it is cheap and easy.I carefully wiped the throttle body with a clean rag only.The tag says don't clean it.Throttle plate is closing properly.I had a leaking waterpump 6 months back so i'll probably replace the CPS when i have time.Right now it is so hot in texas we are only doing what we have to to survive, nothing else. terry